Syed Zeeshan Ali

Tungsten-Based SOI Microhotplates for Smart Gas Sensors

This paper is concerned with the design, fabrication, and characterization of novel high-temperature silicon on insulator (SOI) microhotplates employing tungsten resistive heaters. Tungsten has a high operating temperature and good mechanical strength and is used as an interconnect in high temperature SOI-CMOS processes. These devices have been fabricated using a commercial SOI-CMOS process followed by a deep reactive ion etching (DRIE) back-etch step, offering low cost and circuit integration. In this paper, we report on the design of microhotplates with different diameters (560 and 300 mum) together with 3-D electrothermal simulation in ANSYS, electrothermal characterization, and analytical analysis. Results show that these devices can operate at high temperatures (600degC ) well beyond the typical junction temperatures of high temperature SOI ICs (225degC), have ultralow dc power consumption (12 mW at 600degC), fast transient time (as low as 2-ms rise time to 600degC), good thermal stability, and, more importantly, a high reproducibility both within a wafer and from wafer to wafer. We also report initial tests on the long-term stability of the tungsten heaters. We believe that this type of SOI microhotplate could be exploited commercially in fully integrated microcalorimetric or resistive gas sensors.

Post-CMOS wafer level growth of carbon nanotubes for low-cost microsensors—a proof of concept

Here we demonstrate a novel technique to grow carbon nanotubes (CNTs) on addressable localized areas, at wafer level, on a fully processed CMOS substrate. The CNTs were grown using tungsten micro-heaters (local growth technique) at elevated temperature on wafer scale by connecting adjacent micro-heaters through metal tracks in the scribe lane. The electrical and optical characterization show that the CNTs are identical and reproducible. We believe this wafer level integration of CNTs with CMOS circuitry enables the low-cost mass production of CNT sensors, such as chemical sensors.

Design and simulation of resistive SOI CMOS micro-heaters for high temperature gas sensors

This paper describes the design of doped single crystal silicon (SCS) microhotplates for gas sensors. Resistive heaters are formed by an n+/p+ implantation into a Silicon-On-Insulator (SOI) wafer with a post-CMOS deep reactive ion etch to remove the silicon substrate. Hence they are fully compatible with CMOS technologies and allows for the integration of associated drive/detection circuitry. 2D electro-thermal models have been constructed and the results of numerical simulations using FEMLAB® are given. Simulations show these micro-hotplates can operate at temperatures of 500°C with a drive voltage of only 5 V and a power consumption of less than 100 mW.

Three technologies for a smart miniaturized gas-sensor: SOI CMOS, micromachining, and CNTs - challenges and performance

In this paper we propose a new type of solid-state gas sensor by combining three recent advances, namely silicon-on-insulator CMOS technology, through wafer etching and growth of gas-sensitive carbon nanotubes. We have developed novel tungsten-based CMOS micro-hotplates that offer ultra low power consumption (less than 10 mW at 250degC), on-chip CNT deposition at temperatures up to 700degC, and full integration of CMOS circuitry. Moreover, the tungsten micro-hotplates possess better stability than other CMOS materials such as polysilicon. The multi-walled CNT resistive gas sensors showed a good response to PPB levels of NO2 in air but required additional heating to provide reasonable baseline recovery times. We believe that our approach is attractive for the mass production of low-cost, low-power gas sensors in silicon foundries.

High Performance SOI-CMOS Wall Shear Stress Sensors

Here we present for the first time, a novel silicon on insulator (SOI) complementary metal oxide semiconductor (CMOS) MEMS thermal shear stress sensor for turbulent flow measurements based on aluminum hot-film as a sensing element. These devices have been fabricated using commercial 1 mum SOI-CMOS process followed by a deep reactive ion etch (DRIE) back-etch step, offering low cost and the option of circuit integration. The sensors have a good spatial resolution (size 130 mum times 130 mum) and a very efficient thermal isolation (due to their location on a 500 mum times 500 mum, low thermal conductivity silicon oxide membrane). Results show that these sensors have a high temperature coefficient of resistance (TCR) (0.319%/degC), a low power consumption (below 10 mW for 100degC temperature rise) and a high reproducibility within a wafer and from wafer to wafer. In constant temperature (CT) mode, the sensors exhibit an average sensitivity of 22 mV/Pa in a wall shear stress range of 0-1.5 Pa and an ultra-short time constant of only 17 mus, which corresponds to a high cut-off frequency of 39 kHz.

SOI diode temperature sensor operated at ultra high temperatures - a critical analysis

This paper investigates the performance of diode temperature sensors when operated at ultra high temperatures (above 250degC). A low leakage silicon on insulator (SOI) diode was designed and fabricated in a 1 mum CMOS process and suspended within a dielectric membrane for efficient thermal insulation. The diode can be used for accurate temperature monitoring in a variety of sensors such as microcalorimeters, IR detectors, or thermal flow sensors. A CMOS compatible micro-heater was integrated with the diode for local heating. It was found that the diode forward voltage exhibited a linear dependence on temperature as long as the reverse saturation current remained below the forward driving current. We have proven experimentally that the maximum temperature can be as high as 550degC. Long term continuous operation at high temperatures (400degC) showed good stability of the voltage drop. Furthermore, we carried out a detailed theoretical analysis to determine the maximum operating temperature and explain the presence of nonlinearity factors at ultra high temperatures.

Ultra-high temperature (≫ 300°C) suspended thermodiode in SOI CMOS technology

This paper reports for the first time on the performance and long term continuous operation of a suspended silicon on insulator (SOI) thermodiode with tungsten metallisation at temperatures beyond 300degC. The thermodiode has been designed and fabricated with minute saturation currents (due to both small size and the use of SOI technology) to allow an ultra-high temperature range and minimal nonlinearity. It was found that the thermodiode forward voltage drop vs temperature plot remains linear upto 500degC, with a non-linearity error of less than 7%. Extensive experimental results on performance of the thermodiode, fabricated using a CMOS (complimentary metal oxide semiconductor) SOI process have been presented. These results are backed up by infra red measurements and a range of 2D and 3D simulations using ANSYS and ISE software. The on-chip electronics for thermodiode and micro-heater drive, as well as the transducing circuit for the sensor were placed adjacent to the membrane. Moreover, we demonstrate that the thermodiode is considerably more reliable in long-term direct current operation at high temperatures when compared to the more classical resistive temperature detectors (RTDs) using CMOS metallisation layers (Tungsten or Aluminum). Finally, we believe that the thermodiode suffers less of piezojunction/piezo-resistive effects when compared to silicon based RTDs. For this we compare a membrane thermodiode with a reference thermodiode placed on the silicon substrate and assess their relative performance at elevated temperatures.

A Low-Power, Low-Cost Infra-Red Emitter in CMOS Technology

In this paper, we present the design and characterization of a low-power low-cost infra-red emitter based on a tungsten micro-hotplate fabricated in a commercial 1-μm silicon on insulator-CMOS technology. The device has a 250-μm diameter resistive heater inside a 600-μm diameter thin dielectric membrane. We first present electro-thermal and optical device characterization, long term stability measurements, and then demonstrate its application as a gas sensor for a domestic boiler. The emitter has a dc power consumption of only 70 mW, a total emission of 0.8 mW across the 2.5-15-μm wavelength range, a 50% frequency modulation depth of 70 Hz, and excellent reproducibility from device-to-device. We also compare two larger emitters (heater size of 600 and 1800 μm) made in the same technology that have a much higher infra-red emission, but at the detriment of higher power consumption. Finally, we demonstrate that carbon nanotubes can be used to significantly enhance the thermo-optical transduction efficiency of the emitter.

High-Sensitivity Single Thermopile SOI CMOS MEMS Thermal Wall Shear Stress Sensor

In this paper, we present a novel silicon-on-insulator (SOI) complementary metal-oxide-semiconductor (CMOS) microelectromechanical-system thermal wall shear stress sensor based on a tungsten hot-wire and a single thermopile. Devices were fabricated using a commercial 1-

Silicon-on-Insulator Photodiode on Micro-Hotplate Platform With Improved Responsivity and High-Temperature Application

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